This invention relates to an induction motor control system, and more particularly to improvements in a control circuit including switching elements such as thyristors to control the DC braking torque produced by an induction motor.
There are known induction motor control systems of the type comprising bidirectional thyristors for controlling an induction motor while operating in the running control mode and a rectifier circuit including at least one pair of unidirectional thyristors for controlling the induction motor while operating in the DC braking control mode. At the instant of changing from the running control mode to the DC braking control mode, the induction motor produces an induced voltage thereacross and the unidirectional thyristors have additionally and differentially applied thereacross the induced voltage and the secondary voltage across the rectifier transformer included in the rectifier circuit. As a result, a high direct current flows through the induction motor whose current is abnormally high as compared with the applied control signal for changing the operation of the induction motor from the running control mode to the DC braking control mode. This has consciously occurred particularly when an induction motor being operated in the regenerative control mode is decelerated by changing to the DC braking control mode.
Accordingly it is an object of the present invention to provide an induction motor control system comprising a new and improved control circuit means for controlling the DC braking torque produced by an induction motor so as to suppress the abnormally high direct current as above described to a magnitude which is nearly zero.